1. Field of the Invention
This invention generally relates to a method and an apparatus for detecting distance to an object and more particularly relates to a method and an apparatus for detecting distance to an object with high precision.
2. Description of the Prior Art
Recently, various auto focus cameras which can automatically detect the distance to an object have been developed, and an ultrasonic type auto focus camera is one of such cameras. With the ultrasonic type auto focus camera, when its shutter is released, an ultrasonic wave is directed toward an object from the camera and then the camera receives the ultrasonic wave reflected by the object, whereby the time required by the ultrasonic wave from its emission from the camera until its arrival at the camera again is detected to measure the distance between the camera and the object. Then, a taking or imaging lens incorporated in the camera is suitably moved to automatically focus the camera. Such an ultrasonic type auto focus camera has the advantage that precision of detection of the distance to the object from the camera is not dependent on the brightness and contrast of the object. But, it has a defect in that different results will be brought about depending on whether the object is picked up by the camera from the front or from the oblique side. And when the object is picked up through a window, the distance to the object can not be detected but the distance to the window is detected.
Another type of auto focus camera, a so-called triangulation type camera, generally comprises a fixed mirror and a movable mirror being operated with the taking lens, both being incorporated therein. In the camera, these fixed and movable mirrors are located to reflect light from the front of the camera to two faces of a prism perpendicular to each other, and the light from the fixed mirror and the movable mirror are reflected by the prism and introduced to the light receiving screen of a light receiver. Then, the positional relation between the image of the object from the fixed mirror and the image of the object from the movable mirror, which are both projected onto the light receiving screen of the light receiver, is detected. The movable mirror is rotatably or obliquely moved to bring the positional relation between the images from the two mirrors into a predetermined relation thereby moving the taking lens to the front and rear corresponding to the movement of the movable mirror to focus the camera. In accordance with such an auto focus camera, whether the object is picked up from the front or oblique direction, the focal point can be properly adjusted at all times. Moreover, even when the object is picked up through a window, accurate focusing is possible. However, in the known triangulation type auto focus camera, since the image of the object passed through the taking lens is focussed on a film on the basis of the results of detecting the distance between the object and the camera measured by a rangefinder optical system, which is located separately from the taking lens, parallax will occur. Hence such a camera is quite unsuitable especially for a camera using a zoom lens. Furthermore, this camera also poses a problem in that a special motor is required to drive the movable mirror. In a still picture camera, if a spring or the like is used, the movable mirror can be driven. But, in the case of a camera for animation, where the position of the focal point is varied at all times, a motor must be used for driving the movable mirror, thus giving rise to problems of noise and increases in power consumption, size and cost, none of can be neglected.